Method for forming metal coatings



Feb. 18, 1969 TAKAAKI SHIMOSE ETAL 3,428,472

METHOD FOR FORMING METAL COATINGS Filed Oct. 13, 1964 Sheet of 2 8. 9TAKAAKI SHIMOSE ETAL 3,423,472

METHOD FOR FORMING METAL COATINGS Filed 001;. 15, 1964 Sheet 2 of 2' mm,5;, A J /om [kl/110 fugue BY Z eumH/h FA/ma my w United States Patent3,428,472 METHOD FOR FORMING METAL COATINGS Takaaki Shimose,Nishinomiya, Kenji Mori, Rokkodaimachi, Kobe, Kimio Inoue, Kobe, andZenichiro Takao, Nishinomya, Japan, assignors to Kobe Steel Works, Ltd.,Kobe, Japan Filed Oct. 13, 1964, Ser. No. 403,600 Claims priority,application Japan, Oct. 14, 1963, 38/55,272; Nov. 7, 1963, 38/60,051 US.Cl. 117-22 9 Claims Int. Cl. C23c 3/00; B44d 1/46 ABSTRACT OF THEDISCLOSURE Clean steel strip is coated with a viscous aqueous solutionof potassium polymetaphosphate and sodium polymetaphosphate, and thencontacted with powered metal which is adhesively retained by thepolymetaphosphate solution. The coated strip is dried, compressedbetween rolls, and the powder coating is thereafter sintered. It isbright even without a finishing pass between another pair of rollers,and the composite material has high fieXural strength without spalling.

The present invention relates to an improved method for forming ametallic film on the surface of a metallic :body, the film being of adifferent metal than the metallic body; more specifically this inventionrelates to a method applicable for coating the surface of iron or steelin the form of plate, wire, bar or tube with aluminum, titanium,zirconium, nickel, chromium or copper, or alternatively with an aluminumbase alloy, titanium base alloy, zirconium base alloy or stainlesssteel.

Various processes for forming metallic coatings on metallic bodies arealready known. The most common process is electroplating, butelectroplating is limited to only some types of metals and this methodrequires a relatively long time for forming thick layers. Another knownprocess for forming metallic coatings is the vapor deposition process.This process can be used to form a wide variety of metal coatings, butsince this process has to be carried out in a vacuum, the size of themetal bodies to be coated is subject to some limitation which makes theoperation impractical for a continuous treatment operation. In addition,the film formed by the vapor deposition process is generally quite thin,so this process can not be used when a thick metallic film or coating isdesired. The hot-dip process long employed for forming metallic coatingsis, applicable only to low melting metals. The thermal diffusion processis impractical for continuous operation. The metal spraying process cannot produce high density coatings having uniform thickness.

Accordingly, one object of the present invention is to provide a methodof coating the surface of metallic bodies or substrates with metalsdifferent from the metal bodies to be coated while avoiding thedisadvantages of the prior art methods.

Another object of the present invention is to provide a novel metalcoating method in which powdered materials are employed as coatingmetals.

A still further object of the present invention is to provide productshaving any desired thickness of metal coatings formed thereon.

Patented Feb. 18, 1969 According to the present invention, there isprovided a metal coating method which comprises the steps of applying alayer of powdered coating material onto the surface of a metallic bodyor substrate, pressing the powdered metal against the surface of thesubstrate by mechanical means, and sintering said compressed coatingmetal so as to bond the particles of the coating layer metal to eachother and to the substrate, the bonding of the coating layer to thesubstrate metal being carried out in the presence of an adhesive mediumcomprising an aqueous solution which contains sodium or potassiumpolymetaphosphate.

FIG. 1 is a schematic View of an apparatus suitable for carrying out apreferred method according to the present invention, FIG. 2 is anenlarged cross-sectional view of a steel body carrying a powderedaluminum coating layer formed by the novel method of the presentinvention. FIG. 3 is a schematic view of an apparatus suitable forcarrying out a modified method of the present invention, and FIG. 4 isan enlarged cross-sectional view of a steel body carrying a titaniumcoating layer formed by the method of the present invention.

The metallic substrates which are to be coated by the method of thepresent invention include iron, carbon steel or alloy steel in the formof plate, wire, rod or tube, and the coating materials which are to beemployed as coatings for these substrate materials include aluminum,titanium, zirconium, nickel, chromium, copper, and alloys containing anyone of these materials such as stainless steel. The particle size of thepowdered metal used for coating should be less than 200 mesh.

An aqueous solution of sodium or potassium polymetaphosphate hasexceedingly high viscosity so that a small amount is sufficient toadhere the powdered coating material firmly to the surface of a metallicsubstrate, when the coating material is mechanically pressed against thesurface of the metallic substrate in a subsequent step. Only a smallamount of the dried adhesive material re mains on the coating layer andthis has little or no effect on the bond between the metallic substrateand the coating layer. The polymetaphosphate also accelerates theadhesive action between the metallic substrate and coat ing material.The polymetaphosphate may be either in the form of a chain compound or aring compound, but a chain compound is preferable in the presentinvention because of its high degree of polymerization. Potassiumpolymetaphosphate can be easily polymerized to a high degree but isinsoluble or sparingly soluble in water, and accordingly, the potassiumsalt is generally employed in combination with sodium polymetaphosphateor any other suitable sodium salt.

The degree of polymerization of the polymetaphosphate employed isgenerally within a range from 10 10 However, if the sodium salt isemployed with the potassium salt, the polymerization degree of thesodium salt may be subtsantially lower. The preferred concentration ofthe polyphosphate is 0.01-2.0 g./l. depending upon the polymerizationdegree of the polyphosphate employed.

In carrying out the method of the present invention, the followingconditions may be selectively employed as the situation may require.

The metallic substrate or body to be treated may be in the form ofstrip, plate, wire or any other suitable form.

The powdered metal must be evenly distributed over the surface of thesubstrate as by spraying or by passing the substrate through a fluidizedbed of the coating material. Finely ground coating material the particlesize of which is less than 30 microns may be suspended in the abovementioned adhesive solution prior to its application onto the surface ofthe substrate metal, and the adhesive solution containing the finelypowdered metal coating material may be applied directly to the surfaceof the substrate by spraying, brushing, roll coating or curtain coating.

The bond obtained between the powdered metal and the substrate largelydepends upon the aflinity of the coating material for the adhesivesolution. When the powdered metal has a high affinity for the adhesive,the coating layer generally becomes thick and is liable to cover thesurface of the substrate metal unevenly.

The affinity of powdered metal to the adhesive can be reduced by addinga hydrophobic material.

Materials which impart hydrophobic properties to the powdered metalinclude paraflins having more than 4 carbon atoms, olefins having morethan 5 carbon atoms, aliphatic monohydric alcohols having more than 8carbon atoms, monocarboxylic acids having more than 4 carbon atoms orequivalent polar aliphatic compounds such as monohalide compounds,monocarboxyl compounds, monoamines, and mononitriles.

The amount of the powdered metal that adheres to the substrate decreasesin proportion to the amount of such organic compounds added to thepowder. If desired, the hydrophobic property of the powdered metalcoating material may be reduced by adding a suitable amount of surfaceactive agent or low molecular alcohol to the aqueous solution of theadhesive. The surface active agents suitably employed in the method ofthe present invention include cationic or anionic surface active agents,or nonionic surface active agents, but surface active agents which canbe easily pyrolyzed and volatized are preferably employed.

Although a lower alkanol such as ethyl alcohol or propyl alcohol isemployed in an amount greater (over 5% by volume) than a surface activeagent, the use of alcohol is preferable to the use of the surface activeagent because these alcohols volatize at lower temperatures than thesurface active agents.

After the powdered metal coating has been applied thereon, the metalsubstrate is dried at a temperature ranging from 50 to 450 C. wherebythe powdered metal is caused to adhere firmly to the substrate metalsurface even when the substrate is lightly vibrated. The solvent andadhesive are vaporized during the heat treatment. The heat treatment ordrying temperature at which the adhesive is vaporized should be lowerthan 450 C. in order to prevent oxidation of the powdered metal coatingand metal substrate; the critical temperature varies depending upon thetype of adhesive employed and the length of the heating time.

The step in which the powdered metal coating is mechanically pressedagainst the substrate surface constitutes a pretreatment for thesintering step and the pressing operation may be carried out by means ofroll pressing, die-drawing or roller-die rolling. In any case, thereduction ratio is suitably selected with regard to the particular typeof metal coating powder employed. When a comparatively hard metal powdersuch as titanium is employed as the coating metal, the reduction ratioshould be over 5%, but in case a comparatively soft metal powder such asaluminum is employed as the coating metal, the reduction ratio may be inthe order of 1%.

The mechanical pressing causes the particles of the powdery coatingmetal to adhere to each other and to the metal substrate to form auniform and bright continuous coating.

A mat coating layer may be formed by a rough roll employed in thismechanical pressing, but such a mat coating does not adhere stronglyenough for a final coating layer, so the mat or rough layer is subjectedto a sintering treatment. The sintering treatment is carried out in aninert atmosphere or vacuum at a temperature ranging from 4001100 C. forfive hours. The sintering temperature and time may vary depending uponthe metal coating powder employed. For example, when aluminum is coatedon iron or steel, and the temperature is 600 C., an adherent aluminumlayer is obtained in five minutes, but if the temperature is 550 C., thesame quality of aluminum coating may be obtained in about l030 minutes.This heat treatment serves to sinter the aluminum coating layer and tofirmly adhere the layer onto the metal substrate, but this treatmentshould be carried out in such a manner that no intermediate alloy layeris formed between the aluminum coating layer and metal substrate or, ifany, the thickness of such an alloy layer must be held to less than 3microns. If the heat treatment is carried out at a temperature above 700C. or the treatment continues for several hours, an intermediate alloylayer comprising iron and aluminum is inevitably formed between the ironsubstrate and aluminum layer.

If titanium or zirconium is deposited on iron or steel, and if the heattreatment is carried out at 1000 C., a firmly bonded coating is obtainedin less than 20 minutes while at 850 C. the same quality of coating maybe obtained within 60 minutes.

As is clear from the foregoing, the method of the present inventioncomprises the steps of applying powdered metal onto the surface of ametal substrate in the presence of an adhesive medium comprising anaqueous solution of polymetaphosphate whereby a coating layer of saidpowdered material is formed on the substrate, pressing said coatinglayer against the substrate surface by mechanical means to firmly bondthe coating layer thereto, and sintering said coating layer on thesubstrate surface. The method is applicable to most types of substratemetal and coating metal. The thickness of the coating may be easilycontrolled over a wide range and the coating layer obtained is ofuniform thickness. In addition, this novel coating method can be easilyperformed without unusual facilities or a vacuum room and is suitablefor continuous operation.

For a better understanding of the present invention, several specificexamples are given below.

Example 1 The continuous strip coating line schematically illustrated inFIG. 1 was employed with a mild steel strip S having a. thickness of 0.5mm. and previously cleaned by degreasing and pickling. The pre-treatedmild steel strip S was passed through an aqueous adhesive bath 1 of 1.0g./l. of sodium polymetaphosphate (polymerization degree of 28) and 0.5g./l. of potassium polymetaphosphate (polymerization degree of 10 -10The strip S coated with a thin poly-metaphosphate solution layer ofuniform thickness was then passed under through a funnel shaped hopper 2containing aluminum powder (approximately 50 micron particle size) so asto deposit the aluminum powder on the adhesive-coated surface in a layerof uniform thickness. The thus treated strip S was then passed through adrying kiln 3 maintained at 180 C. and the dried strip S was, thereafterpassed between pairs of opposing rolls 4 set so as to reduce thethickness of the strip coating to 40-50% and thereafter passed through aheating furnace 5 in which argon gas was flowing at 600 C. for oneminute thereby sintering the aluminum powder and bonding the powder morefirmly. Strip S was then passed between another pair of opposing rolls 6to finish the aluminum coating surface and to impart brightness to thesurface. The obtained product had an aluminum coating layer of 30 micronthickness. As shown in FIG. 2, the aluminum coating layer has highflexural strength and can withstand bending with a 3.5 cm. radiuswithout damage to the bond between the metal coating and substrate.

Example 2 In this example, the line schematically illustrated in FIG. 3was employed. Mild steel strip S having a thickness of 0.5 mm. wascleaned by degreasing and pickling before being fed to the line. Thepre-treated strip S was passed through a roll coater containing anaqueous solution of 1.0 g./l. of sodium polymetaphosphate(polymerization degree of 28) and 0.5 g./l. of potassiumpolymetaphosphate. The strip S was coated with an even thickness ofpolymetaphosphate solution while passing through the roll coater 10 andthen passed under a powder applicator -11, containing free-flowingtitanium powder (particle size of about 50 microns) so as to distributethe titanium powder continuously and evenly on the adhesive coatedsurface of the strip which was then passed through a drying kiln 12maintained at 180 C. for about one minute and the dried strip S waspassed through a press 13 set to compress the titanium bearing strip byabout 20%, thereby insuring that the titanium powder firmly adhered tothe strip. After the pressing operation, strip S was wound on a recoiler14 and the coiled strip was heated to 850 C. in a bell-type vacuumheating furnace for one hour. The vacuum was 1 10- mm. Hg.

The thickness of the titanium coating layer was about 30 microns asshown in FIG. 4, and the bond between the metal substrate and coatinglayer was strong enough to withstand bending through 180 at a radius ofcurvature three times its thickness. The appearance of the coatingsurface was fine.

Example 3 A specimen of steel strip was cleaned by degreasing andpickling in the conventional way. The pretreated strip was immersed in asolution of 1 g./l. of potassium polymetaphosphate (polymerizationdegree of 10 -10 and 2 g./l. of sodium polymetaphosphate (polymerizationdegree of 28) and then dusted with stainless steel powder of 325 meshthereby producing a steel strip with a stainless powder coating layer ofuniform thickness. Thereafter, the coated strip was dried and itsstainless coating pressed against the substrate by rollers. The pressedstrip was thereafter heated at 950 C. for about one hour to bond thebright stainless steel coating.

Example 4 20 cc. of a benzene solution containing 0.5 g./dl. of solidparaflin was added to 100 g. of atomized aluminum powder of less than250 mesh and these materials were thoroughly mixed. The mixture was thendried and dusted on the surface of a steel plate covered with a film ofan aqueous solution containing 1.5 g./l. of poly-metaphosphate (0.5g./l. of potassium polymetaphosphate and 1.0 g./l. of sodiumpolymetaphosphate) thereby distributing the aluminum powder evenly onthe surface of the steel plate. The thus coated steel plate was thensubjected to the subsequent steps mentioned in Example 1 and a goodquality of aluminum coated steel plate was obtained.

Example 5 200 cc. of ethanol solution containing 0.125% of a caproicacid was mixed with 1 kg. of atomized aluminum powder of less than 250mesh and the mixture was dried and distributed evenly onto the surfaceof a steel plate previously covered with adhesive solution and furthertreated in the same manner as in Example 4.

Example 6 A mild steel plate having a thickness of 0.5 mm. was cleanedby degreasing and pickling and covered with a film of aqueous adhesivesolution containing a mixture of 1 g./l. of sodium polymetaphosphate and0.5 g./l. of potassium polymetaphosphate and 0.1% of a surface activeagent sold under the trade name Rapizol by Nippon Yushi Co., Ltd.(dialkyl ester of sodium sulfossuccinic acid).

The adhesive-coated steel plate was then evenly dusted with aluminumpowder of less than 325 mesh. The plate was then heated at 350 C. forthree minutes to de compose the surface active agent and then reduced byrolls to 30% of original thickness and finally heated in an argonatmosphere at 600 C. for three minutes. The thickness of the aluminumcoating layer was 50 microns; it made a hood bond with the mild steeland had a bright surface. As an alternative process, an identical steelplate was surface-treated with the same type of adhesive solutionwithout any surface active agent added thereto under the same conditionsas mentioned above in this example. The thickness of the aluminumcoating layer was 20 microns.

What is claimed is:

1. A method of applying a metal coating to a metallic substrate whichcomprises:

(a) coating a surface of said substrate with a viscous aqueous solutionof a polymetaphosphate of potassium or sodium;

(b) contacting the coated substrate with a powdered metal until a layerof metal powder is adhered to said substrate by said solution;

(c) drying the powder bearing substrate at a temperature lower than thesintering temperature of said powder until the water in said solution issubstantialy volatilized;

(d) mechanically pressing said layer against said substrate until thethickness thereof is reduced; and

(e) heating the substrate bearing the pressed layer to said sinteringtemperature until said powder is sintered.

2. A method as set forth in claim 1, wherein said solution contains 0.01to 2.0 g./l. potassium polymetaphosphate having a polymerization degreeof 10 to 10 3. A method as set forth in claim 2, wherein said powderedmetal contains an amount of an organic material sufiicient to imparthydrophobic properties to said powdered metal, and said organic materialis a paraffin having more than 4 carbon atoms or a monocarboxylic acidhaving more than 4 carbon atoms.

4. A method as set forth in claim 2, wherein said viscous solutioncontains 0.01-0.5% of a surface active agent.

5. A method as set forth in claim 2, wherein said viscous solutioncontains at least 5% by volume of a lower alkanol.

6. A method as set forth in claim 2, wherein said substrate is ferrous,said sintering temperature is between 400 and 1100 C., and said powderis sintered in an inert atmosphere for less than five hours.

7. A method as set forth in claim 6, wherein said powdered metal isaluminum, titanium, zirconium, nickel, chromium, or copper, and theparticle size of said powdered metal is smaller than 200 mesh.

8-. A method of applying a metal coating to a metallic substrate whichcomprises:

(a) suspending a metal powder having a particle size of less than 30microns in an aqueuos solution of a polymetaphosphate of potassium orsodium;

'(b) applying the suspension so produced in a uniform layer to a surfaceof said substrate;

(c) substantially removing the water of said solution from said layeruntil the latter is substantially dry;

(d) mechanically pressing the metal powder of the dry layer against saidsurface until the thickness of said layer is reduced; and

(e) heating the substrate bearing the pressed layer to the sinteringtemperature of said metal powder until the same is sintered.

9. A method as set forth in claim 8, wherein said metal powder isaluminum, titanium, zirconium, nickel, chromium or copper, water isremoved from said layer by heating the same to a temperature between 50and 7 8 450 C., and said metal powder is sintered at 400 to 2,858,60011/1958 Vigor 117-451 X 1100 C. for less than five hours. 2,895,192v7/1959 Meissner 11750 X 3,024,128 3/1962 Dawson 117-22 References Cited3,142,560 7/1964 Storchheirn 117-22 UNITED STATES PATENTS 3,182,3955/1965 SCOtt 11722 X 1 7 5 11/1929 Dely 117 22 X 5 WILLIAM D. MARTIN,Primary Examiner. 1,922,254 8/ 1933 McCulloch 11722 X PAUL A'ITAGUILE,Assistant Examiner. 2,190,237 2/1940 Koehring 11722 2,261,228 11/1941Cockrum 117-22 2,289,614 7/1942 Wesley et a1. 117-22 10 117-33,130

